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<title>Transformers</title>

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</head>

<body>




<h1 class="title toc-ignore">Transformers</h1>



<p>Transformers allow you to apply functions to the glue input and output, before and after evaluation. This allows you to write things like <code>glue_sql()</code>, which automatically quotes variables for you or add a syntax for automatically collapsing outputs.</p>
<p>The transformer functions simply take two arguments <code>text</code> and <code>envir</code>, where <code>text</code> is the unparsed string inside the glue block and <code>envir</code> is the execution environment. Most transformers will then call <code>eval(parse(text = text, keep.source = FALSE), envir)</code> which parses and evaluates the code.</p>
<p>You can then supply the transformer function to glue with the <code>.transformer</code> argument. In this way users can manipulate the text before parsing and change the output after evaluation.</p>
<p>It is often useful to write a <code>glue()</code> wrapper function which supplies a <code>.transformer</code> to <code>glue()</code> or <code>glue_data()</code> and potentially has additional arguments. One important consideration when doing this is to include <code>.envir = parent.frame()</code> in the wrapper to ensure the evaluation environment is correct.</p>
<p>Some example implementations of potentially useful transformers follow. The aim right now is not to include most of these custom functions within the <code>glue</code> package. Rather, users are encouraged to create custom functions using transformers to fit their individual needs.</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">library</span>(glue)</span></code></pre></div>
<div id="collapse-transformer" class="section level3">
<h3>collapse transformer</h3>
<p>A transformer which automatically collapses any glue block ending with <code>*</code>.</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb2-1"><a href="#cb2-1"></a>collapse_transformer &lt;-<span class="st"> </span><span class="cf">function</span>(<span class="dt">regex =</span> <span class="st">&quot;[*]$&quot;</span>, ...) {</span>
<span id="cb2-2"><a href="#cb2-2"></a>  <span class="cf">function</span>(text, envir) {</span>
<span id="cb2-3"><a href="#cb2-3"></a>    collapse &lt;-<span class="st"> </span><span class="kw">grepl</span>(regex, text)</span>
<span id="cb2-4"><a href="#cb2-4"></a>    <span class="cf">if</span> (collapse) {</span>
<span id="cb2-5"><a href="#cb2-5"></a>      text &lt;-<span class="st"> </span><span class="kw">sub</span>(regex, <span class="st">&quot;&quot;</span>, text)</span>
<span id="cb2-6"><a href="#cb2-6"></a>    }</span>
<span id="cb2-7"><a href="#cb2-7"></a>    res &lt;-<span class="st"> </span><span class="kw">identity_transformer</span>(text, envir)</span>
<span id="cb2-8"><a href="#cb2-8"></a>    <span class="cf">if</span> (collapse) {</span>
<span id="cb2-9"><a href="#cb2-9"></a>      <span class="kw">glue_collapse</span>(res, ...)  </span>
<span id="cb2-10"><a href="#cb2-10"></a>    } <span class="cf">else</span> {</span>
<span id="cb2-11"><a href="#cb2-11"></a>      res</span>
<span id="cb2-12"><a href="#cb2-12"></a>    }</span>
<span id="cb2-13"><a href="#cb2-13"></a>  }</span>
<span id="cb2-14"><a href="#cb2-14"></a>}</span>
<span id="cb2-15"><a href="#cb2-15"></a></span>
<span id="cb2-16"><a href="#cb2-16"></a><span class="kw">glue</span>(<span class="st">&quot;{1:5*}</span><span class="ch">\n</span><span class="st">{letters[1:5]*}&quot;</span>, <span class="dt">.transformer =</span> <span class="kw">collapse_transformer</span>(<span class="dt">sep =</span> <span class="st">&quot;, &quot;</span>))</span>
<span id="cb2-17"><a href="#cb2-17"></a><span class="co">#&gt; 1, 2, 3, 4, 5</span></span>
<span id="cb2-18"><a href="#cb2-18"></a><span class="co">#&gt; a, b, c, d, e</span></span>
<span id="cb2-19"><a href="#cb2-19"></a></span>
<span id="cb2-20"><a href="#cb2-20"></a><span class="kw">glue</span>(<span class="st">&quot;{1:5*}</span><span class="ch">\n</span><span class="st">{letters[1:5]*}&quot;</span>, <span class="dt">.transformer =</span> <span class="kw">collapse_transformer</span>(<span class="dt">sep =</span> <span class="st">&quot;, &quot;</span>, <span class="dt">last =</span> <span class="st">&quot; and &quot;</span>))</span>
<span id="cb2-21"><a href="#cb2-21"></a><span class="co">#&gt; 1, 2, 3, 4 and 5</span></span>
<span id="cb2-22"><a href="#cb2-22"></a><span class="co">#&gt; a, b, c, d and e</span></span>
<span id="cb2-23"><a href="#cb2-23"></a></span>
<span id="cb2-24"><a href="#cb2-24"></a>x &lt;-<span class="st"> </span><span class="kw">c</span>(<span class="st">&quot;one&quot;</span>, <span class="st">&quot;two&quot;</span>)</span>
<span id="cb2-25"><a href="#cb2-25"></a><span class="kw">glue</span>(<span class="st">&quot;{x}: {1:5*}&quot;</span>, <span class="dt">.transformer =</span> <span class="kw">collapse_transformer</span>(<span class="dt">sep =</span> <span class="st">&quot;, &quot;</span>))</span>
<span id="cb2-26"><a href="#cb2-26"></a><span class="co">#&gt; one: 1, 2, 3, 4, 5</span></span>
<span id="cb2-27"><a href="#cb2-27"></a><span class="co">#&gt; two: 1, 2, 3, 4, 5</span></span></code></pre></div>
</div>
<div id="shell-quoting-transformer" class="section level3">
<h3>Shell quoting transformer</h3>
<p>A transformer which automatically quotes variables for use in shell commands, e.g. via <code>system()</code> or <code>system2()</code>.</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb3-1"><a href="#cb3-1"></a>shell_transformer &lt;-<span class="st"> </span><span class="cf">function</span>(<span class="dt">type =</span> <span class="kw">c</span>(<span class="st">&quot;sh&quot;</span>, <span class="st">&quot;csh&quot;</span>, <span class="st">&quot;cmd&quot;</span>, <span class="st">&quot;cmd2&quot;</span>)) {</span>
<span id="cb3-2"><a href="#cb3-2"></a>  type &lt;-<span class="st"> </span><span class="kw">match.arg</span>(type)</span>
<span id="cb3-3"><a href="#cb3-3"></a>  <span class="cf">function</span>(text, envir) {</span>
<span id="cb3-4"><a href="#cb3-4"></a>    res &lt;-<span class="st"> </span><span class="kw">eval</span>(<span class="kw">parse</span>(<span class="dt">text =</span> text, <span class="dt">keep.source =</span> <span class="ot">FALSE</span>), envir)</span>
<span id="cb3-5"><a href="#cb3-5"></a>    <span class="kw">shQuote</span>(res)</span>
<span id="cb3-6"><a href="#cb3-6"></a>  }</span>
<span id="cb3-7"><a href="#cb3-7"></a>}</span>
<span id="cb3-8"><a href="#cb3-8"></a></span>
<span id="cb3-9"><a href="#cb3-9"></a>glue_sh &lt;-<span class="st"> </span><span class="cf">function</span>(..., <span class="dt">.envir =</span> <span class="kw">parent.frame</span>(), <span class="dt">.type =</span> <span class="kw">c</span>(<span class="st">&quot;sh&quot;</span>, <span class="st">&quot;csh&quot;</span>, <span class="st">&quot;cmd&quot;</span>, <span class="st">&quot;cmd2&quot;</span>)) {</span>
<span id="cb3-10"><a href="#cb3-10"></a>  .type &lt;-<span class="st"> </span><span class="kw">match.arg</span>(.type)</span>
<span id="cb3-11"><a href="#cb3-11"></a>  <span class="kw">glue</span>(..., <span class="dt">.envir =</span> .envir, <span class="dt">.transformer =</span> <span class="kw">shell_transformer</span>(.type))</span>
<span id="cb3-12"><a href="#cb3-12"></a></span>
<span id="cb3-13"><a href="#cb3-13"></a>}</span>
<span id="cb3-14"><a href="#cb3-14"></a></span>
<span id="cb3-15"><a href="#cb3-15"></a>filename &lt;-<span class="st"> &quot;test&quot;</span></span>
<span id="cb3-16"><a href="#cb3-16"></a><span class="kw">writeLines</span>(<span class="dt">con =</span> filename, <span class="st">&quot;hello!&quot;</span>)</span>
<span id="cb3-17"><a href="#cb3-17"></a></span>
<span id="cb3-18"><a href="#cb3-18"></a>command &lt;-<span class="st"> </span><span class="kw">glue_sh</span>(<span class="st">&quot;cat {filename}&quot;</span>)</span>
<span id="cb3-19"><a href="#cb3-19"></a>command</span>
<span id="cb3-20"><a href="#cb3-20"></a><span class="co">#&gt; cat &#39;test&#39;</span></span>
<span id="cb3-21"><a href="#cb3-21"></a><span class="kw">system</span>(command)</span></code></pre></div>
</div>
<div id="emoji-transformer" class="section level3">
<h3>emoji transformer</h3>
<p>A transformer which converts the text to the equivalent emoji.</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb4-1"><a href="#cb4-1"></a>emoji_transformer &lt;-<span class="st"> </span><span class="cf">function</span>(text, envir) {</span>
<span id="cb4-2"><a href="#cb4-2"></a>  <span class="cf">if</span> (<span class="kw">grepl</span>(<span class="st">&quot;[*]$&quot;</span>, text)) {</span>
<span id="cb4-3"><a href="#cb4-3"></a>    text &lt;-<span class="st"> </span><span class="kw">sub</span>(<span class="st">&quot;[*]$&quot;</span>, <span class="st">&quot;&quot;</span>, text)</span>
<span id="cb4-4"><a href="#cb4-4"></a>    <span class="kw">glue_collapse</span>(<span class="kw">ji_find</span>(text)<span class="op">$</span>emoji)</span>
<span id="cb4-5"><a href="#cb4-5"></a>  } <span class="cf">else</span> {</span>
<span id="cb4-6"><a href="#cb4-6"></a>    <span class="kw">ji</span>(text)</span>
<span id="cb4-7"><a href="#cb4-7"></a>  }</span>
<span id="cb4-8"><a href="#cb4-8"></a>}</span>
<span id="cb4-9"><a href="#cb4-9"></a></span>
<span id="cb4-10"><a href="#cb4-10"></a>glue_ji &lt;-<span class="st"> </span><span class="cf">function</span>(..., <span class="dt">.envir =</span> <span class="kw">parent.frame</span>()) {</span>
<span id="cb4-11"><a href="#cb4-11"></a>  <span class="kw">glue</span>(..., <span class="dt">.open =</span> <span class="st">&quot;:&quot;</span>, <span class="dt">.close =</span> <span class="st">&quot;:&quot;</span>, <span class="dt">.envir =</span> .envir, <span class="dt">.transformer =</span> emoji_transformer)</span>
<span id="cb4-12"><a href="#cb4-12"></a>}</span>
<span id="cb4-13"><a href="#cb4-13"></a><span class="kw">glue_ji</span>(<span class="st">&quot;one :heart:&quot;</span>)</span>
<span id="cb4-14"><a href="#cb4-14"></a><span class="kw">glue_ji</span>(<span class="st">&quot;many :heart*:&quot;</span>)</span></code></pre></div>
</div>
<div id="sprintf-transformer" class="section level3">
<h3>sprintf transformer</h3>
<p>A transformer which allows succinct sprintf format strings.</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb5-1"><a href="#cb5-1"></a>sprintf_transformer &lt;-<span class="st"> </span><span class="cf">function</span>(text, envir) {</span>
<span id="cb5-2"><a href="#cb5-2"></a>  m &lt;-<span class="st"> </span><span class="kw">regexpr</span>(<span class="st">&quot;:.+$&quot;</span>, text)</span>
<span id="cb5-3"><a href="#cb5-3"></a>  <span class="cf">if</span> (m <span class="op">!=</span><span class="st"> </span><span class="dv">-1</span>) {</span>
<span id="cb5-4"><a href="#cb5-4"></a>    format &lt;-<span class="st"> </span><span class="kw">substring</span>(<span class="kw">regmatches</span>(text, m), <span class="dv">2</span>)</span>
<span id="cb5-5"><a href="#cb5-5"></a>    <span class="kw">regmatches</span>(text, m) &lt;-<span class="st"> &quot;&quot;</span></span>
<span id="cb5-6"><a href="#cb5-6"></a>    res &lt;-<span class="st"> </span><span class="kw">eval</span>(<span class="kw">parse</span>(<span class="dt">text =</span> text, <span class="dt">keep.source =</span> <span class="ot">FALSE</span>), envir)</span>
<span id="cb5-7"><a href="#cb5-7"></a>    <span class="kw">do.call</span>(sprintf, <span class="kw">list</span>(<span class="kw">glue</span>(<span class="st">&quot;%{format}f&quot;</span>), res))</span>
<span id="cb5-8"><a href="#cb5-8"></a>  } <span class="cf">else</span> {</span>
<span id="cb5-9"><a href="#cb5-9"></a>    <span class="kw">eval</span>(<span class="kw">parse</span>(<span class="dt">text =</span> text, <span class="dt">keep.source =</span> <span class="ot">FALSE</span>), envir)</span>
<span id="cb5-10"><a href="#cb5-10"></a>  }</span>
<span id="cb5-11"><a href="#cb5-11"></a>}</span>
<span id="cb5-12"><a href="#cb5-12"></a></span>
<span id="cb5-13"><a href="#cb5-13"></a>glue_fmt &lt;-<span class="st"> </span><span class="cf">function</span>(..., <span class="dt">.envir =</span> <span class="kw">parent.frame</span>()) {</span>
<span id="cb5-14"><a href="#cb5-14"></a>  <span class="kw">glue</span>(..., <span class="dt">.transformer =</span> sprintf_transformer, <span class="dt">.envir =</span> .envir)</span>
<span id="cb5-15"><a href="#cb5-15"></a>}</span>
<span id="cb5-16"><a href="#cb5-16"></a><span class="kw">glue_fmt</span>(<span class="st">&quot;π = {pi:.2}&quot;</span>)</span>
<span id="cb5-17"><a href="#cb5-17"></a><span class="co">#&gt; π = 3.14</span></span></code></pre></div>
</div>
<div id="safely-transformer" class="section level3">
<h3>safely transformer</h3>
<p>A transformer that acts like <code>purrr::safely()</code>, which returns a value instead of an error.</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb6-1"><a href="#cb6-1"></a>safely_transformer &lt;-<span class="st"> </span><span class="cf">function</span>(<span class="dt">otherwise =</span> <span class="ot">NA</span>) {</span>
<span id="cb6-2"><a href="#cb6-2"></a>  <span class="cf">function</span>(text, envir) {</span>
<span id="cb6-3"><a href="#cb6-3"></a>    <span class="kw">tryCatch</span>(</span>
<span id="cb6-4"><a href="#cb6-4"></a>      <span class="kw">eval</span>(<span class="kw">parse</span>(<span class="dt">text =</span> text, <span class="dt">keep.source =</span> <span class="ot">FALSE</span>), envir),</span>
<span id="cb6-5"><a href="#cb6-5"></a>      <span class="dt">error =</span> <span class="cf">function</span>(e) <span class="cf">if</span> (<span class="kw">is.language</span>(otherwise)) <span class="kw">eval</span>(otherwise) <span class="cf">else</span> otherwise)</span>
<span id="cb6-6"><a href="#cb6-6"></a>  }</span>
<span id="cb6-7"><a href="#cb6-7"></a>}</span>
<span id="cb6-8"><a href="#cb6-8"></a></span>
<span id="cb6-9"><a href="#cb6-9"></a>glue_safely &lt;-<span class="st"> </span><span class="cf">function</span>(..., <span class="dt">.otherwise =</span> <span class="ot">NA</span>, <span class="dt">.envir =</span> <span class="kw">parent.frame</span>()) {</span>
<span id="cb6-10"><a href="#cb6-10"></a>  <span class="kw">glue</span>(..., <span class="dt">.transformer =</span> <span class="kw">safely_transformer</span>(.otherwise), <span class="dt">.envir =</span> .envir)</span>
<span id="cb6-11"><a href="#cb6-11"></a>}</span>
<span id="cb6-12"><a href="#cb6-12"></a></span>
<span id="cb6-13"><a href="#cb6-13"></a><span class="co"># Default returns missing if there is an error</span></span>
<span id="cb6-14"><a href="#cb6-14"></a><span class="kw">glue_safely</span>(<span class="st">&quot;foo: {xyz}&quot;</span>)</span>
<span id="cb6-15"><a href="#cb6-15"></a><span class="co">#&gt; foo: NA</span></span>
<span id="cb6-16"><a href="#cb6-16"></a></span>
<span id="cb6-17"><a href="#cb6-17"></a><span class="co"># Or an empty string</span></span>
<span id="cb6-18"><a href="#cb6-18"></a><span class="kw">glue_safely</span>(<span class="st">&quot;foo: {xyz}&quot;</span>, <span class="dt">.otherwise =</span> <span class="st">&quot;Error&quot;</span>)</span>
<span id="cb6-19"><a href="#cb6-19"></a><span class="co">#&gt; foo: Error</span></span>
<span id="cb6-20"><a href="#cb6-20"></a></span>
<span id="cb6-21"><a href="#cb6-21"></a><span class="co"># Or output the error message in red</span></span>
<span id="cb6-22"><a href="#cb6-22"></a><span class="kw">library</span>(crayon)</span>
<span id="cb6-23"><a href="#cb6-23"></a><span class="kw">glue_safely</span>(<span class="st">&quot;foo: {xyz}&quot;</span>, <span class="dt">.otherwise =</span> <span class="kw">quote</span>(<span class="kw">glue</span>(<span class="st">&quot;{red}Error: {conditionMessage(e)}{reset}&quot;</span>)))</span>
<span id="cb6-24"><a href="#cb6-24"></a><span class="co">#&gt; foo: Error: object &#39;xyz&#39; not found</span></span></code></pre></div>
</div>
<div id="variables-and-values-transformer" class="section level3">
<h3>“Variables and Values” transformer</h3>
<p>A transformer that expands input of the form <code>{var_name=}</code> into <code>var_name = var_value</code>, i.e. a shorthand for exposing variable names with their values. This is inspired by an <a href="https://docs.python.org/3.8/whatsnew/3.8.html#f-strings-now-support-for-quick-and-easy-debugging">f-strings feature coming in Python 3.8</a>. It’s actually more general: you can use it with an expression input such as <code>{expr=}</code>.</p>
<div class="sourceCode" id="cb7"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb7-1"><a href="#cb7-1"></a>vv_transformer &lt;-<span class="st"> </span><span class="cf">function</span>(text, envir) {</span>
<span id="cb7-2"><a href="#cb7-2"></a>  regex &lt;-<span class="st"> &quot;=$&quot;</span></span>
<span id="cb7-3"><a href="#cb7-3"></a>  <span class="cf">if</span> (<span class="op">!</span><span class="kw">grepl</span>(regex, text)) {</span>
<span id="cb7-4"><a href="#cb7-4"></a>    <span class="kw">return</span>(<span class="kw">identity_transformer</span>(text, envir))</span>
<span id="cb7-5"><a href="#cb7-5"></a>  }</span>
<span id="cb7-6"><a href="#cb7-6"></a></span>
<span id="cb7-7"><a href="#cb7-7"></a>  text &lt;-<span class="st"> </span><span class="kw">sub</span>(regex, <span class="st">&quot;&quot;</span>, text)</span>
<span id="cb7-8"><a href="#cb7-8"></a>  res &lt;-<span class="st"> </span><span class="kw">identity_transformer</span>(text, envir)</span>
<span id="cb7-9"><a href="#cb7-9"></a>  n &lt;-<span class="st"> </span><span class="kw">length</span>(res)</span>
<span id="cb7-10"><a href="#cb7-10"></a>  res &lt;-<span class="st"> </span><span class="kw">glue_collapse</span>(res, <span class="dt">sep =</span> <span class="st">&quot;, &quot;</span>)</span>
<span id="cb7-11"><a href="#cb7-11"></a>  <span class="cf">if</span> (n <span class="op">&gt;</span><span class="st"> </span><span class="dv">1</span>) {</span>
<span id="cb7-12"><a href="#cb7-12"></a>    res &lt;-<span class="st"> </span><span class="kw">c</span>(<span class="st">&quot;[&quot;</span>, res, <span class="st">&quot;]&quot;</span>)</span>
<span id="cb7-13"><a href="#cb7-13"></a>  }</span>
<span id="cb7-14"><a href="#cb7-14"></a>  <span class="kw">glue_collapse</span>(<span class="kw">c</span>(text, <span class="st">&quot; = &quot;</span>, res))</span>
<span id="cb7-15"><a href="#cb7-15"></a>}</span></code></pre></div>
<div class="sourceCode" id="cb8"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb8-1"><a href="#cb8-1"></a><span class="kw">set.seed</span>(<span class="dv">1234</span>)</span>
<span id="cb8-2"><a href="#cb8-2"></a>description &lt;-<span class="st"> &quot;some random&quot;</span></span>
<span id="cb8-3"><a href="#cb8-3"></a>numbers &lt;-<span class="st"> </span><span class="kw">sample</span>(<span class="dv">100</span>, <span class="dv">4</span>)</span>
<span id="cb8-4"><a href="#cb8-4"></a>average &lt;-<span class="st"> </span><span class="kw">mean</span>(numbers)</span>
<span id="cb8-5"><a href="#cb8-5"></a>sum &lt;-<span class="st"> </span><span class="kw">sum</span>(numbers)</span>
<span id="cb8-6"><a href="#cb8-6"></a></span>
<span id="cb8-7"><a href="#cb8-7"></a><span class="kw">glue</span>(<span class="st">&quot;For {description} {numbers=}, {average=}, {sum=}.&quot;</span>, <span class="dt">.transformer =</span> vv_transformer)</span>
<span id="cb8-8"><a href="#cb8-8"></a><span class="co">#&gt; For some random numbers = [28, 80, 22, 9], average = 34.75, sum = 139.</span></span>
<span id="cb8-9"><a href="#cb8-9"></a></span>
<span id="cb8-10"><a href="#cb8-10"></a>a &lt;-<span class="st"> </span><span class="dv">3</span></span>
<span id="cb8-11"><a href="#cb8-11"></a>b &lt;-<span class="st"> </span><span class="fl">5.6</span></span>
<span id="cb8-12"><a href="#cb8-12"></a><span class="kw">glue</span>(<span class="st">&quot;{a=}</span><span class="ch">\n</span><span class="st">{b=}</span><span class="ch">\n</span><span class="st">{a * 9 + b * 2=}&quot;</span>, <span class="dt">.transformer =</span> vv_transformer)</span>
<span id="cb8-13"><a href="#cb8-13"></a><span class="co">#&gt; a = 3</span></span>
<span id="cb8-14"><a href="#cb8-14"></a><span class="co">#&gt; b = 5.6</span></span>
<span id="cb8-15"><a href="#cb8-15"></a><span class="co">#&gt; a * 9 + b * 2 = 38.2</span></span></code></pre></div>
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